During World War Two the fighter gun reached the peak of its
importance, as at that time it was the only effective weapon available for
air­to-air fighting. This period saw it developed to a dramatic extent, from the
rifle-calibre machine guns which were standard at the start of the war to
powerful automatic cannon of 20 mm to 50 mm calibre by the end. However,
aircraft guns were increasingly challenged by missiles in the post-war period.
At first, the moves to replace guns with missiles were premature, as the
Americans discovered during the Vietnam War. Since the 1970s, the performance of
air­to-air and air-to-ground guided missiles has improved dramatically, to the
degree that it is frequently argued that there is little reason to fit guns to
aircraft in the future.

The Royal Air Force's initial decision not to use
the Mauser BK 27 cannon in its Eurofighters is a sign of the times.
Furthermore, only the United States Air Force (USAF) version of the
Lockheed Martin F-35 Joint Strike Fighter (JSF) will have an internal
cannon, the United States Navy (USN) and short take-off/vertical
landing (STOVL) versions being available only with a gun-pod as an
option. Although the Lockheed Martin F/A-22 Raptor does have an
integral cannon, one senior staff member in charge of the project has
since expressed regret about the decision.

There is now a strong body of opinion which regards aircraft
guns as troublesome and unnecessary. The cost of providing space and weight for
the gun, acquiring and maintaining it, and keeping the pilot's skills up to
date, are all cited. Vibration, resulting from gun firing, can affect avionics
and cause structural fatigue, and propellant residues, which coat the aircraft
are possibly corrosive. However, in spite of all this there are still good
reasons to retain guns, as we shall see.

The case for the gun 1: air
combat

Modern short-range missiles have minimum ranges as low as 300
m, well within gun range. They are also highly agile, with wide engagement
envelopes, enabling them to hit targets well off bore-sight (the missile's
centre-line), especially when cued by a helmet-mounted sight: in fact, the
capabilities of most recent missiles are such that the aircraft carrying them
barely need to manoeuvre. This does not mean that guns are useless for
air-to-air work. They have a particular value in modern 'policing' applications,
as they enable warning tracer shots to be fired in front of suspect aircraft.
They also provide an economical way of engaging low value targets such as
unmanned reconnaissance drones, transport and liaison aircraft, or
drug-smugglers. In combat they still carry certain potential advantages in
close-quarter fighting, for example in 'picking off' an enemy attacking a
wingman who may be too close for a safe missile shot. The ability of modern
fighters to adopt extreme attitudes, pointing well away from the line of flight,
significantly assists gun aiming in dogfights. Cannon are instantly ready to
fire, and their projectiles have a shorter flight time than a missile.

Finally, the gun provides a last-ditch capability when all of
the missiles have been fired, or if they are defeated by advanced
countermeasures, or simply by circumstances. The 1991 Gulf War revealed the
deficiencies of modern infra-red (IR) homing missiles when faced with trying to
pick up a low-flying target against a hot desert background (helicopters being
in any case difficult for IR seekers to lock on to from above). USAF Fairchild
A-10 Thunderbolt II ground-attack aircraft achieved two helicopter kills with
the GAU-8/A cannon, in one case when the IR missiles failed to lock on.
Furthermore, the performance of even the best missiles cannot always be
guaranteed. In Kosovo, a US fighter engaging a Serbian aircraft needed to fire
three Advanced Medium Range Air-to-Air Missiles (AMRAAMs) to bring it down. In
other engagements during the late 1990s, USAF and USN aircraft fired a total of
seven AIM-7 Sparrows, AMRAAMs, and AIM-54 Phoenix missiles against Iraqi MiG-25
Foxbats without scoring a single hit (although the Phoenix shots were
taken at extreme range).

In part, the low success rates are due to tactical
considerations, in that missiles may deliberately be launched outside the normal
engagement envelope to distract or scare off the enemy, and sometimes two
missiles are launched at one target to increase the hit probability. Whatever
the reason, this results in missiles being used up at a high rate, making it
more likely that they will run out during a sortie. A cannon will typically
carry enough ammunition for several engagements, usefully increasing combat
persistence at a minimal cost in weight and performance.

Of the 1,000 kills or so
achieved between 1958 and 1991, only a handful were scored beyond visual range (BVR),
which raises questions about the significance of very long range missiles. It is
sometimes argued that modern short-range missiles are so good that any aircraft
with the benefit of long-range sensors and missiles should use them to try to
stay outside the envelope of the enemy's short-range AAMs. However, it is not
always possible to dictate the terms of an engagement.
The Iranians made good use of the long-range AIM-54
in their 1980s war with Iraq, but the F-14s which carried it still found
themselves engaged in gunfights from time to time.

There is a continual battle between missile sensor and
countermeasure technology. In the future, stealth technology applied to aircraft
may considerably shorten target acquisition and combat ranges, putting into
question the worth of modern BVR (beyond visual range) AAMs. The possible future
use of anti-radar missile homing as a way of overcoming stealth characteristics
may force fighters to make minimal use of their own radars, further reducing
acquisition and combat distances. It may also prove increasingly difficult for
either IR or radar-homing missiles to lock on to their stealthy targets,
additionally protected by extensive electronic jamming and IR countermeasures.

Modern guns are usually aimed by the aircraft's radar, which
could also be jammed (though less easily than the much smaller and less-powerful
missile seekers), but laser rangefinders could make an acceptable alternative in
providing fire control data. If aircraft eventually become 'laser­proof' as
well, the possibility presumably exists of linking variable-magnification
optical sights to a computer which would be able to analyse the image, identity
the aircraft, calculate its distance, speed and heading and provide gunsight-aiming
information accordingly, all covertly.

The case for the gun 2: ground
attack

The emphasis in the use of aircraft guns has now shifted more
towards air-to-ground work, although even this is becoming increasingly
hazardous. The proliferation of anti-aircraft gun and missile systems, including
man-portable air defence systems (MANPADS), means that even specialist
ground-attack aircraft, fitted with powerful cannon, have found it safer to rely
on the long range of their air-to-ground guided weapons rather than close to gun
range, although the USAF's A-10s have still made good use of their cannon in
recent conflicts.

This trend is aided by the continued development of
air-to-surface missiles, the latest ones having autonomous homing systems to
provide 'fire and forget' capability over long ranges. Another current
development is the General Dynamics Advanced Precision Kill Weapon System, which
aims to achieve accuracy at low-cost by fitting a laser homer to the small 2¾
inch (70 mm) rocket. The target is to achieve a circular error probable of l-2 m
at ranges of up to 5-6 km at a price of US$8-10,000; one-sixth the cost of a
Hellfire anti-tank missile.

However, not all conflicts involve front­line opposition: in
fact, armed forces are now commonly engaged on police work, frequently dealing
with guerrilla forces. In these circumstances, rockets and missiles may
represent an inappropriate degree of destruction, with a high risk of collateral
damage. The RAF was embarrassed during operations against insurgents in Sierra
Leone in 2000 to find that it had no suitable weapon for its (gunless) BAE
Systems Harrier GR.7s to attack small groups of rebels operating close to
innocent civilians. Guns may also have value in providing the option to fire
warning shots or inflict limited damage on land or naval targets in a display of
determination.

Another advantage of using cannon was demonstrated during the
invasion of Afghanistan in 2002. Air support was called for during an intense
infantry battle at Takur Ghar in late May, in which US forces were ambushed and
in considerable danger. The Lockheed AC-130 Hercules gunship was not permitted
to intervene in daylight due to its vulnerability, so USAF fighters were sent to
help. For part of the battle the Afghan combatants were too close to the
American forces for rockets or bombs to be used, so the fighters - Lockheed
Martin F-16s and even Boeing F-15 Eagles - went in strafing with their 20 mm
cannon, as have the Navy's F-14s and F/A-18 Hornets on other occasions in recent
conflicts. Even RAF Tornados are reported to have carried out gun-strafing runs
on at least one occasion.

Which Gun?

If an aircraft gun is still worth its cost and weight, should
it be built in or added as a bolt-on gunpod when required? What type of weapon
is best? What calibre should it be?

Podded guns have an advantage in that they do not need to be
carried unless the aircraft are in circumstances in which a gun is likely to be
needed. Like any other piece of weaponry, they can be fitted according to
requirements. The downside of this is the need to make sure that the gunpods are
available when required. This needs careful planning and an element of luck.
They occupy a hard point which would otherwise be available for fuel or other
weapons, they take some time to harmonise - and keep harmonised - when fitted,
and even then they are less accurate than integral guns. Gunpods generate more
drag, usually affect handling, and are also much less 'stealthy' than integral
guns - a factor likely to increase in importance as the growing use of stealth
measures is leading to the internal carriage of all weapons. The gunpod is
therefore very much second-best.

There are three competing philosophies when it comes to gun
design for fighter aircraft. One is the US multi-barrel rotary in the form of
the classic M61, now in A2 form in the F/A-18E/Fand F/A-22: despite
being fast-firing, it is only 20 mm in calibre, and a very bulky system. This is
supplemented (mainly for ground attack) by the use of the five-barrel 25 mm
GAU-12/U, fitted as a podded gun attached to the USMC's
McDonnell Douglas AV-8B Harrier II. A four-barrel version of this gun,
the GAU-22/A, is to be fitted internally to the Air Force Lockheed Martin F-35A
Lightning II and as a gunpod to the STOVL F-35 and naval F-35C. The
seven-barrel 30 mm GAU-8/A is the most powerful aircraft gun currently in
service, but, together with its ammunition magazine and feed, it is so large and
heavy that its only user, the A-10 close-support aircraft, had to be designed
around it and it cannot be considered as a general­purpose aircraft gun. A
smaller version firing the same ammunition, the four-barrel GAU-13/A, is
available only in the GPU-5/A gunpod, but the vibration of the powerful gun has
been proved to significantly degrade its accuracy.

The second approach is the West European preference
for a 27 mm – 30 mm revolver cannon, with multiple firing chambers but
a single barrel. For many decades the standard weapons have been the
French and British twins: the 30mm DEFA/GIAT and Aden revolver cannon,
directly derived from the Mauser MK 213 under development at the end of
World War Two, and fitted in various forms to a wide range of aircraft.
The GIAT was also developed further in South Africa as the Vektor 55C5,
but this did not enter production. A larger and appreciably more
powerful option was the 30 mm Oerlikon KCA, fitted only (in a conformal
pod) to fighter versions of the Saab Viggen (now out of service), but
also available in a detachable gunpod. In terms of muzzle energy; this
is still the most powerful fighter gun: the GAU-8/A 'tankbuster'
ammunition was developed from it. Later still came the 27 mm Mauser BK
27, a good combination of compactness, hitting power and high rate of
fire. Its qualities led to its selection by both Boeing and Lockheed
Martin for the JSF competition, but the choice for the winning F-35 was
subsequently switched to the GAU12/U before the GAU-22/A was finally
chosen. The most recent development is the GIAT 30M791, so far intended
only for the Dassault Rafale. This combines a powerful new 30 mm
cartridge with a very high rate of fire.

For the sake of completeness, it should be mentioned that the
USA made one revolver cannon which saw considerable use in the 1950s - the M39,
which fired the same 20 mm ammunition as the M61. It was eclipsed in the 1960s
by the M61 and by now would have been long forgotten if not for its use in the
F-5 lightweight fighter, still in service in some numbers.

The third philosophy, adopted by Russia, is a variety of
light and compact weapons (the penalty being a relatively short life). Their
principal fighter gun (designed, like all of them, by the partnership of Gryazev
and Shipunov) is the 30 mm GSh-301 as used in the MiG-29 Fulcrum and
Su-27 Flanker families. This has a remarkable performance considering it
uses an 'old-fashioned' single-chamber short-recoil operation, similar in
principle to the Browning machine gun of World War Two. It matches the BK 27 in
performance, but weighs only half as much.

The twin-barrel GSh-30 also deserves mention, despite not
having been used in fighters, being restricted to the Su-25 Frogfoot
attack aircraft (and, in long-barrelled GSh-30K form, to the Mil Mi-24P Hind
combat helicopter). At 105 kg it weighs about the same as the Western
fighter guns, but fires at up to 3,000 rpm. It was originally intended to be
fitted to the MiG-29 and Su-27, until displaced by the GSh-301.

Finally, the third gun to use the same powerful 30 mm
ammunition is the six-barrel rotary GSh­6-30, which differs from the American
rotaries in being gas-operated rather than externally powered, giving it a
faster 'spin-up' time. This was also restricted to ground attack, in the
MiG-27 Flogger, though in comparison with the GAU-8/A is considerably
lighter and more compact, being not much bigger or heavier than the 25 mm
GAU-12/U. The battering this powerful gun gave the relatively light MiG-27
caused many problems, however, and it is no longer in service (see THIS article).

The Russians also use two guns in the much less powerful 23
mm calibre: the twin-barrel GSh-23 and the six-barrel rotary GSh-6-23. The
GSh-23 is one of the classic aircraft guns, having been used in a wide variety
of aircraft since emerging in the late 1950s. By comparison with its Western
equivalent, the M61A1, it fires only half as fast, but it weighs less than half
as much and is far more compact. It has recently been modified in the Czech
Republic to use American 20 mm ammunition; the resulting ZPL-20 is available
only in a gun pod.

The gas-operated GSh-6-23 has seen relatively little use,
being installed only in the MiG-31 Foxhound and the SPPU-6 gunpod.
However, it sets the world record in rate of fire for an aircraft cannon of no
less than 9,000+ rpm, and is considerably smaller and lighter than the M61AI.

An interesting comparison can be made between the two
principal Western fighter guns, the M61A1 and the BK 27. The rotary clearly has
the advantage in rate of fire, but it fires much smaller and lighter shells.
Another difference between the two is that the externally-powered rotary gun
takes time to spin up to its maximum rate of fire, whereas the revolver has an
instant response. In the first 0.5 second, the M61 fires 18 rounds totalling 1.8
kg: whereas the BK 27 fires 14 rounds weighing over 3.7 kg. In the first second,
the M61 fires 68 rounds weighing 6.9 kg, the BK 27 fires 28 rounds weighing over
7.4 kg. In weight of fire, as well as the destructiveness of the individual
projectiles, the Mauser clearly has an advantage, albeit one that the
faster-accelerating M61A2 reduces somewhat. This is significant in that
dogfights frequently permit only the briefest of firing opportunities, and
although a skilled pilot anticipating a firing opportunity can 'spin up' a
rotary in advance, such notice cannot always be guaranteed. The Mauser
projectiles are also heavier, which means they will retain their initial
velocity out to a greater range. Incidentally, the choice of an even heavier
projectile fired at a moderate velocity for the Russian 30 mm guns implies that
ground attack has a higher priority than aerial combat in Russian strategy.

The ideal gun for aerial combat will, of course, combine the
best of all worlds: a high rate of fire, instantly achieved; a high muzzle
velocity to minimise flight time; and projectiles large enough to inflict
serious damage with each hit (requiring a calibre in the 25 mm – 30 mm range).
The optimum weapon among those currently developed may well be the new GIAT
30M791 revolver, although its weight means that two GSh-301s (or a GSh-30) could
be carried instead, with a higher rate of fire. If the Russian guns' 30 x 165
(shell diameter and cartridge case length in mm) ammunition were loaded with
lighter projectiles for a higher muzzle velocity, their aerial combat
capabilities would be improved, at the cost of some loss of ground attack
effectiveness.

For general ground attack (as opposed to 'tankbusting'), the
Russian 30 x 165 round is ideal, and there are three weapons capable of firing
it, with one, two or six barrels. Which is best depends on-the limitations on
weight, space and 'recoil management' of the aircraft concerned.

Future of the aircraft gun

All the weapons so far described represent conventional, if
not rather old, technology, The USAF has been exploring new technologies for
aircraft guns under various programmes ever since World War Two. The 25mm
GAU-7/A, firing combustible-cased telescoped ammunition (CTA) and originally
intended for the F-15, was a technical failure but the concept has reappeared in
a new range of CTA weapons under development in both the USA and Europe. Most
attention is being paid to armoured fighting vehicles guns in 40 mm – 45 mm
calibre but a 20 mm aircraft gun project has also featured. In the 1980s, the
USAF Advanced Gun Technology programme led to a General Electric design for a
five-barrel, 20 mm gun with less than half the number of parts of the M61. The
gun weighed 102 kg, had a maximum firing rate of 6,000 rpm and a muzzle velocity
of 1,525 m/s, a speed which provides the potential to achieve a huge increase in
hit probability.

Another advanced development might conceivably include the
adoption of Oerlikon AHEAD (advanced hit efficiency and destruction) technology
- time-fuzed shrapnel shells which shower the target with a shotgun blast of
tungsten-alloy sub-munitions. A simple version of this is already in service for
ground attack in the Russian CC projectiles, but its application to air combat
would involve resolving some difficult fire control problems in that the time
fuze is set electronically at the muzzle to detonate the shell just ahead of the
target.

If, in the future, guns were to regain some of their
importance in air combat, there are technologies available which could
significantly improve the hit probability. An example is the 'closed loop'
control system used in the naval Phalanx close-in weapon system, in which the
gun-aiming radar also tracks the projectiles in flight, calculating whether they
will hit the target and adjusting the aim if not. Applied to an aircraft, this
could operate in three ways: the aim could be adjusted by steering the aircraft
(linking gun firing to a transfer of flying control to the flight control
system), by steering a flexibly­mounted gun (the Soviets were keen on this in
the 1950s, but with only manual aiming it was too cumbersome and would not be
very stealthy) or by steering the projectiles (course-correctable cannon shells
were demonstrated decades ago, using small charges around the circumference of
the projectile to alter its trajectory).

Looking further ahead, the possibility exists of using
electromagnetic rail guns, which use a powerful pulse of electromagnetic energy
to accelerate a projectile to velocities higher than any chemical propellant can
achieve. However, some fundamental technical breakthroughs would be necessary to
make this feasible in an aircraft weapon.

Work on airborne lasers is also proceeding. Initially these
will be huge weapons carried by large transport aircraft for destroying
ballistic missiles during their ascent phase, but fighter installations are also
being considered. Raytheon is working on solid-state lasers for this purpose,
with the possibility of using fibre optics later. The original aim was to mount
such a weapon on the F-35 by 2010. The beam could be steered by mirrors in any
direction, potentially a very valuable ability in a dogfight. Such a system
could conceivably also be used to provide an anti­missile self-defence
capability for the fighter. However, a laser is susceptible to atmospheric
conditions and, no doubt, countermeasures will also be developed, perhaps
involving ablative coatings which not only absorb the
heat of a laser strike but produce a highly-reflective surface when hit.

These approaches are all years - if not decades - away from
being ready for service use. Until then, the versatility in the kind of targets
which can be engaged, flexibility (in terms of a progressive response from
warning shots up to destruction), the lack of susceptibility to atmospheric
conditions, stealth technology and countermeasures, and the low cost of gun
munitions, all combine to make the retention of the aircraft gun a strong
probability for the foreseeable future. Some recent decisions – namely, that the
BK 27 will, after all, be carried by all RAF Eurofighters (initially set up only
for ground attack), indicate that this
has been recognised.

For a more technical analysis of modern
aircraft guns and their ammunition, click HERE